A self-consistent model for the long-term gamma-ray spectral variability of Cygnus X-1

Fulvio Melia, Ranjeev Misra

Research output: Contribution to journalArticlepeer-review

20 Scopus citations


The long-term transitions of the black hole candidate Cygnus X-1 (between the states γ1, γ2, and γ3) include the occasional appearance of a strong ∼ MeV bump (γ1), whose strength appears to be anticorrelated with the continuum flux (≲ 400 keV) due to the Compton upscattering of cold disk photons by the inner, hot corona. We develop a self-consistent disk picture that accounts naturally for these transitions and their corresponding spectral variations. We argue that the bump is due to the self-Comptonization of bremsstrahlung photons emitted predominantly near the plane of the corona itself. Our results suggest that a decrease by a factor of ≈ 2 in the viscosity parameter α is responsible for quenching this bump and driving the system to the γ2 state, whereas a transition from γ2 to γ3 appears to be induced by an increase of about 25% in the accretion rate Ṁ. In view of the fact that most of the transitions observed in this source seem to be of the γ23 variety, we conclude that much of the long-term gamma-ray spectral variability in Cygnus X-1 is due to these small fluctuations in Ṁ. The unusual appearance of the γ1 state apparently reflects a change in the dissipative processes within the disk.

Original languageEnglish (US)
Pages (from-to)797-801
Number of pages5
JournalAstrophysical Journal
Issue number2
StatePublished - Jul 10 1993


  • Accretion, accretion disks
  • Black hole physics
  • Radiation mechanisms: miscellaneous
  • Stars: individual (Cygnus x-1)

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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